An image acquisition system may be used in motor vehicles in order to obtain images of the vehicle's surroundings and, in connection with a driver assistance system, to make it easier for the driver to manage the vehicle. An image acquisition system of this kind encompasses at least one image sensor and an optical system, associated with said image sensor, that images onto the image sensor an imaged field of the vehicle's surroundings. One task of any such driver assistance system is precise measurement of distance, since it is only with a knowledge of accurate distance values that optically based lane and distance monitoring systems, having functions such as, for example, lane departure warning (LDW) and lane keeping support (LKS), can function with sufficient reliability. Conventionally, image acquisition systems using two cameras that generate a stereo image pair of an object may be used, the contents of one image being slightly shifted with respect to the other image. This shift is referred to as a disparity. If the arrangement of the cameras is known, the distance of the object from the cameras can be deduced from the measured disparity. With an image acquisition system having only one image sensor, however, the disparity can no longer be readily determined, since no stereo image is generated. The image sensors used with image acquisition systems of this kind must process a wide range of illumination intensities so that they can still supply usable output signals on the one hand in bright sunlight and on the other hand in dimly illuminated tunnels. Whereas with conventional image sensors the exposure sensitivity often follows a preset linear or logarithmic characteristic curve, image sensors have been described, for example, in German Patent Application No. DE 103 01 898 A1, in which this characteristic curve is individually adjustable in individual linear segments. A characteristic curve of this kind correlates the absolute brightness of an object with the grayscale value in the image obtained of the object.
German Patent No. DE 4332612 A1 describes an external viewing method for motor vehicles that is characterized by the following steps: acquiring an external view from the driver's own motor vehicle, which is moving; sensing a motion of an individual point in two images as an optical flow, one of the two images being acquired at an earlier point in time and the other of the two images at a later point in time; and monitoring a correlation of the driver's own motor vehicle with respect at least to either a preceding motor vehicle or an obstacle on the road, a hazard level being evaluated as a function of a magnitude and a location of a vector of an optical flow that is derived from a point on at least the preceding motor vehicle, the following motor vehicle, or the obstacle on the road. Taking into account the fact that the optical flow becomes greater as the distance from the driver's own vehicle to the preceding motor vehicle or obstacle becomes smaller, or as the relative velocity becomes greater, this conventional method is therefore designed so that the hazard can be evaluated on the basis of the magnitude of an optical flow that is derived from a point on a preceding motor vehicle or on an obstacle on the road. It is therefore not particularly necessary to install a distance measuring instrument in order to measure the distance to a preceding motor vehicle.